We propose to design and build a new slit-scan chromosomal flow cytometer specifically for the high-speed quantitative analysis of DNA content, centromere location and centromere number of isolated human chromosomes. Our current cell-oriented system can measure the DNA content of one thousand fluorescently stained human chromosomes per second at coefficients of variation of 2 percent. The new system will operate at the same rate and with much tighter control of the trajectory and velocity of the chromosomes; in a major departure from previous design. It will excite fluorescence as the oriented chromosomes flow lengthwise through the narrow dimension of a thin ribbon of laser light, and will collect and process a time-dependent fluorescence profile from each chromosome. A dip in the profile will be used to identify and locate the centromere for metacentric chromosomes larger than human l3. DNA content will be measured with a coefficient of variation of 2 percent or less and this, in conjunction with the centromeric index, will identify and give the relative frequency of 20 of the 24 human chromosome types. Detection of dicentric and multicentric chromosomes will also provide data on the incidence of a representative class of chromosomal aberrations. The evolving performance of the machine will be tested on uniform microspheres, rod-shaped abalone sperm, and in chromosomes of the Chinese hamster, muntjac, and human - whose centromeric properties can be predicted from their DNA content.